(1) Field of the Invention
This invention relates generally to electronic display devices and relates more specifically to methods and apparatus for driving electronic matrix display panels.
(2) Description of the Prior Art
Many electronic devices, such as mobile telephones and other portable devices, make use of display panels for displaying information to a user of the device. A schematic diagram of such an electronic device is shown in FIG. 1 prior art of the accompanying drawings. The device 1 comprises a central processing unit (CPU) 10, which receives user and other inputs from an input device 12. The input device 12 may provide a keypad, a pointing device, a microphone, a wireless interface equipment or any other appropriate input apparatus. The CPU 10 operates to store data in a memory device 14, and in a data storage device 16 for non-volatile data storage. The CPU 10 supplies image data to a display device 18 for display to a user. Since embodiments of the present invention are concerned with methods and apparatus associated with the display device, the exemplary structure of an electronic device shown in FIG. 1 prior art should not be considered restrictive or limiting in any way. Other configurations of electronic device may make use of display devices.
The display device 18 includes a display panel 182, which is operable to display an image, and a driver 184, which controls the display panel 182, as will be described in more detail below. There are several types of display panel currently available. However, the display panel of special interest in the present case is a panel provided by a matrix of light emitting devices, such as an electro luminescent panel provided by a passive matrix organic light emitting diode (PM OLED) device. For the sake of clarity, the background and principles of the present invention will be described with reference to a PM OLED display panel, but it will be readily appreciated that the principles are more generally applicable to matrix display panels.
FIG. 2 prior art of the accompanying drawings illustrates schematically a display device 20. The display device 20 comprises a PM OLED display panel 22, and a display driver 24. The display panel 22 comprises a matrix of organic light emitting diodes (OLEDs) 222. By way of background explanation, an OLED is typically made up of two layers of organic material, one of which is a layer of light emitting material such as a light emitting polymer (LEP), oligomer, or a light emitting low molecular weight material, and the other of which is a layer of a hole transporting material such as a polythiophene derivative or a polyaniline derivative. The organic material is deposited onto a substrate material, such as glass, to provide a matrix of light emitting pixels. For a single color display device, a single OLED of the desired color is used per pixel, whereas in a multi color display device three OLEDs per pixel are provided to give red, green and blue output light respectively. The example shown in FIG. 2 prior art is a single color display for the sake of clarity, but it will be readily appreciated that the description below can be applied to a multi color device having red, green and blue OLEDs for each pixel.
The matrix of OLEDs 222 of FIG. 2 prior art, can have any appropriate number of columns and rows of OLEDs 222, but has been shown with four of each. Columns 0, 1, 2 . . . x, and cross rows 0, 1, 2 . . . y to create the matrix. In the following description of operation of the panel, an individual OLED is described as OLED 222-ab, where “a” is a column identifier, and “b” is a row identifier. Each column of the matrix is provided with a column electrode 224 (for example 224-0 for column 0, and 224-x for column x), to which one electrode of each of the OLEDs 222 in that column is connected. Similarly, each row of the matrix is provided with a row electrode 226 (for example 226-1 for row 1, and 226-y for row y), to which a second electrode of each OLED in the row is connected. In the example of FIG. 2 prior art, the anode of each OLED is connected to the appropriate column electrode 224, and the cathode of each OLED is connected to the appropriate row electrode 226. It will be readily appreciated that the OLED electrodes may be connected in an opposite configuration.
In a passive matrix (PM) OLED display panel, the OLEDs are repeatedly driven to give the impression of a steady image being displayed. The driving of an OLED is achieved by the supply of electrical current to the OLED. The current magnitude and length of time of the current supply together determine the brightness of the output light produced by the OLED. It is therefore necessary to provide driving control circuitry to generate the correct current/time values. The display device 20 of FIG. 2 prior art includes a column driver 242 and a row driver 244. In order to drive the OLEDs in the matrix to provide an output image, a voltage is applied across the row and column electrodes by the column driver 242 and the row driver 244, so that drive current is supplied to the OLEDs, and the OLEDs output light. In one previous technique for driving OLEDs, the column driver 242 includes a plurality of constant current supplies, and the row drivers operate to connect the row electrodes in turn to ground, thereby allowing current to flow through selected rows of OLEDs. The column driver 242 can operate to switch the current supplied selectively, so that an individual pixel can be driven by a correct selection of the column and row values. It is known to vary the brightness of the output light of an OLED by varying the time that the current is allowed to flow, and by varying the size of that current. The most usual known technique is to apply a constant current for a variable time, using a so-called “pulse width modulation” (PWM) technique.
Existing known techniques for driving matrix display devices are either simple, but inefficient, or are undesirably complex in the pursuit of efficiency. It is, therefore, desirable to provide a technique for driving a matrix display device that is relatively straightforward, and that provides good efficiency. Specifically, it desirable to provide a technique that can reduce the overall magnitude of power needed to display an image and that can balance peak current and drive time period considerations.
There are known patents or patent publications dealing with drivers for OLED displays.
U.S. Patent Publication (US 2010/0207920 to Chaji et al.) discloses a display system, a driver for driving the display array, method of operating the display system, and a pixel circuit in the display system. The driver includes: a bidirectional current source having a convertor coupling to a time-variant voltage for converting the time-variant voltage to the current. The pixel circuit includes: a transistor for providing a pixel current to a light emitting device; and a storage capacitor electrically coupling to the transistor, the capacitor coupling to a time-variant voltage in a predetermined timing for providing a current based on the time-variant voltage. The pixel circuit includes: an organic light emitting diode (OLED) device having an electrode and an OLED layer; and an inter-digitated capacitor, used for the driving capacitor, having a plurality of layers.
U.S. Patent Publication (US 2010/0103203 to Choi) proposes an organic light emitting display capable of minimizing power consumption. The organic light emitting display includes a plurality of pixels positioned at intersections of data lines and scan lines, the pixels including driving transistors positioned in an effective display region to control an amount of current that flows from a first power source to a second power source, a data driver for supplying data signals to the data lines, a scan driver supplying scan signals to the scan lines, a first power source generator generating the first power source, a second power source generator generating the second power source, and a voltage controller controlling the second power source generator so that voltage of the second power source is changed in response to a first voltage applied to an organic light emitting diode (OLED) included in a specific pixel when a data signal corresponding to specific brightness is supplied from the data driver to the specific pixel.
U.S. Patent Publication (US 2009/0284511 to Tagasuki) discloses an image display wherein luminance change due to change of the light-emitting device over time is compensated while suppressing affects of characteristics change in the drive transistor. Specifically disclosed is an image display comprising a plurality of pixels, wherein each pixel has a light-emitting device (OLED) which emits light when current is passed there through, a driver device for controlling light emission of the light-emitting device, and a control circuit, which is electrically connected to the light-emitting device and the driver device, and directly or indirectly detects the voltage applied to the light-emitting device at least during when the light-emitting device is emitting light and reflects the detection results to the driver device.